1. Field of the Invention
The present invention relates to abrasion resistant topcoatings applied to the surface of liquid permeable control release coatings which in turn encapsulate water soluble substrates to provide controlled or delayed release of the substrate when contacted with water.
2. Description of Related Art
The application of coating materials to the surface of various granular chemical substrates is a well known technique for controlling or delaying the release of that substrate when placed in an environment in which the substrate would otherwise quickly dissolve. Encapsulation of a substrate such as a pharmaceutical, a fertilizer or reagent used in subterranean oil recovery operations using an encapsulant which is slowly permeable to water insulates the substrate from immediate contact with an aqueous environment into which it is introduced, e.g., wet soil as in the case of fertilizer or aqueous fracturing fluids used in fossil oil recovery operations. Over a period of time in contact with the aqueous environment, the substrate is caused to be gradually released into the environment as a consequence of the gradual permeation of water through the encapsulant and into the substrate and gradual leaching away of the dissolved substrate into the environment as a consequence of the build up of osmotic pressure or rupture of the protective coating.
Specific applications of this technology include the use of controlled release oxidizers or breaker chemicals in combination with high viscosity fracturing fluids which are pumped into well bores to improve the efficiency of well drilling operations. A high viscosity, aqueous based fracturing fluid is pumped into the well bore at a rate and pressure sufficient to produce one or more subterranean fractures or extend or enlarge existing fractures. Included in the fracturing fluid is an oxidizer chemical, such as ammonium persulfate, encapsulated in a polymeric coating which is slowly permeable to water. As water permeates the coating and begins to dissolve the substrate, a corresponding build up in osmotic pressure leads to reverse osmosis or a rupture of the coating shell and subsequent release of the oxidizer. Subterranean release of the oxidizer initiates a breakdown in the viscosity of the surrounding fracturing fluid which facilitates removal of the fluid and recovery of residual oil from the enlarged fractures. Such systems are disclosed in U.S. Pat. Nos. 4,741,401 and 4,770,796.
Another application of controlled release technology is the use of water permeable coatings for fertilizer and/or plant nutrient substrates. Rapid release of water soluble plant food in soils containing the plants can cause phytotoxicity to the plants and rapid depletion of the plant food by leaching. This problem is dealt with by encapsulating the fertilizer or nutrient substrate with a thin layer of a material which is slowly permeable to water such that the rate at which the fertilizer is leached into the soil is diminished, and ideally approximates the rate of plant uptake of these nutrients.
Control release coatings which have been used with fertilizer substrates include molten sulfur, paraffin waxes, various polymer coatings such as vinyl or vinylidene chloride copolymers such as disclosed in U.S. Pat. No. 4,711,059, polyurethanes as disclosed in U.S. Pat. Nos. 3,264,089, 4,711,659, and 4,772,490, phenol formaldehyde resins, as disclosed in U.S. Pat. No. 3,744,987, elastomeric polymers such as neutralized, sulfonated ethylene-propylene-diene (EPDM) terpolymers disclosed in U.S. Pat No. 4,988,377 and ionomer polymers such as disclosed in U.S. Pat. No. 4,970,117.
One of the major problems associated with many controlled release coatings such as those generally described above is damage to the thin, fragile coating as a consequence of product handling operations which include blending, packaging, transport and application in the field using mechanical equipment. When damage occurs to a controlled release coating, the product loses its controlled release properties completely or in part, since coating holes, cracks or thin spots allow for rapid access of water in which the substrate material is soluble.